Why did UK and France not declare war on the USSR

This question have buggered me for a few years, and I never seem to get a satisfactory explanation. The UK and France made a guarantee to Poland in the event of an attack from a hostile neighbor. The 1st of September Germany attacked Poland. War declarations followed on the 3rd and 4th September, by these two Great Powers of Europe.

On the 17th, USSR rolls into Eastern Poland and seals the nations fate. But wait! Weren’t UK and France obliged to come to the defense of Poland whoever attacked, so why no declaration of war against the USSR?


Good question, the key is the Treaty of Versailles with garanteed the existence of Poland in accordance to the 14 points plan. The Germans were signees and the Poles basically functioned as the Eastern threat to Germany with France being on the West.

The Soviets did not sign the Versailles treaty but the British did help in the early 1920s Soviet-Polish war. When the Soviets invaded in 1939 it was not mandatory to invade although the Allies considered military action against the Soviets.

Oh and a heartfelt welcome to the forum, hope you’ll enjoy!


It was also evident just from Mein Kampf alone that Hitler intended to turn the East in “living space” so to speak. The British eventually came to recognize that war between Germany and Russia was inevitable and that Russia would be a valuable ally in that regard. Thus it was a “lesser of two evils” approach. History Matters on YouTube has an episode that answers this exact question. You should check him out, highly recommend.


I also think that Hitler was driven by, maybe with some kind of madness, a Roman Empire building attitude with a long time perspective - unfortunately, the historical timing of his life made ww2 possible


The Soviets hadn’t yet directly attacked the UK or France so they probably thought they could kick that can down the road & focus on Germany for the time being, though plans had been drawn up by the British to bomb the oil production at Baku. If you thought Spring 1940 went bad for the Allies in real life, just imagine what would have happened if Soviet forces were there too. Would the British have been able to escape Dunkirk in such a scenario? Remember, the UK also didn’t declare war on Japan until they attacked Hong Kong. In the end, the Western Allies kicked the can long enough for Hitler to invade the Soviets.


If you go on You Tube you’ll get a video covering this topic on the channel Knowledgia. It was the first video I watched about the topic and it gave me great insights into the hows and whys of that whole ordeal.

France and Poland signed a treaty of friendship in 1921 after the Polish-Soviet war which stated that if one or the other were the victim of an unprovoked attack, the other would help.
In May 1939 Poland and France signed a new treaty binding the militaries of the two nations, but this treaty only named Germany as a potential aggressor, thus France was technically not obligated to declare war on the Soviet Union after the Soviets invaded Poland.

The treaty signed between Poland and the United Kingdom however referred to “hostilities with a European power”.
Britain and Poland then also signed a treaty in August 1939 promising mutual military assistance if either were attacked by a European country. This means Britain was technically obligated to go to war with the USSR under the terms of the August 1939 treaty.

However, this was signed by the British hoping to discourage German expansionist ideals.
Thus, a secret clause referred to an attack by Germany specifically.
However if the aggressor was a different European national then Britain and Poland would only to consult together on measures to be taken in common.
Still, under the terms of the pact Britain was legally obligated to declare war on the Soviet Union.

When the Soviet Union invaded the Polish ambassador to London did request assistance.
Lord Halifax said the following: “As regards Soviet aggression we were free to take our own decision and to decide whether to declare war on the USSR or not.”

Still, the British used the secure clause of the 1939 pact to justify not going to war with the Soviet Union saying that they only guaranteed Poland’s protection in case of German aggression.

But fun fact, the Polish was not aware of this secret clause. The British did consider making it public to explain their lack of action to the Poles, but they decided not to. The British also said in October 1939 that the Poles should have understood that the pact only covered Poland in case of German aggression.

British ambassador to Moscow wrote in a secret telegram on 18 September 1939
“I do not myself see what advantage a war with the Soviet Union would be to us…our war aims are not incompatible with reasonable settlement (in Poland) on ethnographic and cultural lines.”

There was also the fact that neither Britain nor France were really prepared for war. They declared war on Germany but I mean neither of them really did much of anything, hence the Phoney War.
Seeing this the USSR thought that the Allies wouldn’t make another empty declaration of war as with Germany, so they thought it was safe to invade Poland without repercussions.

Plus Germany and the Soviet Union were historic rivals, so many people in Britain and France thought that maybe they’ll pick a fight with each other later on.
They also knew about the non-aggression pact between Germany and the Soviet Union, but they figured it was not a military alliance and if they declared war on the Soviet Union it might result in a German-Soviet alliance.
Should this happen Germany would have the natural resources from the Soviet Union as well as their own large industry.

Pre-war the focus was mainly on Germany and their expansion. Neither Britain nor France cared much about the Soviet Union. Britain and France weren’t ready for a war with even Germany, so they didn’t want to add the Soviet Union to the list of enemies.

So France wasn’t technically obligated to declare war on the Soviet Union, but Britain could’ve legally declared war. But in the end the British did not.
Britain and France wanted to stop Germany. They didn’t necessarily want to protect Poland specifically, just stop Germany. They had no designs on the Soviet Union.
Their treaties were to stop German aggression, not Soviet aggression. One can even argue they didn’t care that much about Polish independence as long as Germany stopped its expansion.


This is a really interesting question. My personal opinion is a little simpler and maybe uninformed. I think they didn’t do so for two reasons:

  1. Russia was not adjacent to France and thus did not immediately threaten it. This is after 9/1 of course.
  2. Taking on Russia, who was seen as a sort of friend, after already taking on Germany would be too much to take on. Russia may have used an excuse such as to protect ethnic Slavs in Poland and not as a land grab (which of course it was) as it was underway anyway they let it go.

Welcome to the forum. And yes, often it is a good point to look for logic and simplicity. Also I think the scary experience from WW1 where a string of Alliances brought every to war with each other must have been on their minds.